Add visibility field for npm's pose landmarks

[lin826]

According to the doc: https://developers.google.com/mediapipe/solutions/vision/pose_landmarker/web_js

This should be supported.

Related to #4409 and #4605

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[lin826]

Just a follow-up: Are there any internal concerns or obstacles to supporting this field?

[schmidt-sebastian]

I am working on this but this is breaking a lot of our tests so the change is much bigger than in this PR.